U.S. Pat. No. 4,394,481 is incorporated herein by reference. This patent discloses a chemically modified asphalt (CMA) which is produced by the long-time reaction, under reflux, of an asphalt, a vinyl aromatic monomer, a rubbery polymer and an acrylamide. Such chemically modified asphalts are disclosed for use as protective coatings for various substrates and are applied, for example, to a cementitious substrate from a solvent carrier, typically a mixture of 80% toluene and 20% normal hexane. U.S. Pat. No. 4,507,365 proposes the use of an environmentally desirable solvent mixture of at least 75% cyclohexane, 15 to 2% toluene, and up to 10% n-hexane for essentially the same CMA.
U.S. Pat. No. 4,456,633 provides a chip seal emulsion of essentially the same CMA in an emulsion which may be cationic, anionic or non-ionic. Such emulsions are intended for use only as an additive for conventional non-chemically modified asphalt emulsions to increase the retention of aggregate, for example, when the combined emulsion is utilized for highway pavement repair. The method of use includes blending the CMA emulsion into the non-CMA emulsion, applying the combination to a pavement, and then applying an aggregate to the applied combination emulsion.
U.S. Pat. No. 4,419,489, also incorporated herein by reference, discloses essentially the same CMA incorporated into an aqueous emulsion of the cationic rapid set type, the final emulsion comprising 60-65% CMA and 35-40% aqueous medium. The emulsion was intended for use as a cold overlay or as a joint sealer for highways or as a water-resistant membrane. This type of emulsion has been used as a waterproofing basement coating.
Prior to the present invention, it has not been possible to form a satisfactory aqueous emulsion of a blend of a synthetic elastomer, e.g., a block copolymer of styrene and butadiene. Even where it has been possible to form an emulsion of an asphalt-radial teleblock copolymer blend, the emulsion cures to a residue film which has physical properties and low temperature flexing characteristics significantly less than the equivalent properties of the asphalt-copolymer blend alone.
These prior art emulsions all incorporate, as a thickener, or as a viscosity control, a cellulose-based additive, such as hydroxyethyl cellulose, in order to obtain the desired final emulsion viscosity, i.e., on the order of 4,000 to 8,000 cps at normal ambient temperatures of about 70.degree. F.
Aqueous asphalt emulsions generally cure by evaporation of a water content, leaving a residual film comprising the asphaltic component, the emulsifier or surfactant, and any viscosity control agent. It has been found that the prior art emulsions containing hydroxyethyl cellulose form a dark surface layer or "skin" upon curing, and this skin materially retards water evaporation from the underlying portions of the emulsion. Consequently, a surface coating of a prior art emulsion on a vertical substrate, such as a basement exterior wall, will "skin over" with the underlying uncured emulsion, if overly thick, running down the wall. Similarly, a crack filler emulsion of the prior art will "skin over" and the underlying, still-liquid, uncured emulsion will "pump out" of the underlying crack when subjected to traffic. As a result, the prior art emulsions necessarily require unduly long cure times, and running or "pumping" remains a problem.
The present invention now provides an aqueous emulsion of either a chemically modified asphalt or a blend of asphalt and an elastomer with an aqueous emulsification medium containing a high molecular weight acrylamide as a viscosity control ingredient. It has been found that such emulsions possess desirable physical properties, good low temperature flexing characteristics, and cure more quickly without "skinning over." Only a minute amount of the acrylamide is required, preferably on the order of from about 0.5 to about 10% of a 2% aqueous solution of the acrylamide. Thus, the final preferred emulsification medium will contain from about 0.01% to about 0.20% of the acrylamide.
While the mechanism of the present invention is not fully understood, the previously used hydroxyethyl cellulose apparently thickened the emulsion by its absorption of water. The water-swollen hydroxyethyl cellulose of the prior art apparently materially disrupted the residue film formed upon curing of the emulsion, while the water entrapped by the hydroxyethyl cellulose simply could not escape through the surface skin formed by the initial evaporation of water from the emulsion surface. The relatively large amount of hydroxyethyl cellulose required (on the order of 5 to 100 times as much as the amount of acrylamide required by the present invention, particularly when swollen by the absorbed water) apparently disrupted the blended asphalt emulsion film and, when present at the surface from which evaporation was taking place, caused the emulsion to "skin over."
In contrast, the acrylamide of this invention is of extremely high molecular weight, and the acrylamide apparently is a true thixotropic agent which serves to gel the emulsion when it rests, while the emulsion acts as a liquid under agitation or pressure, thereby accommodating spraying of the emulsion as a coating or dispensing of the emulsion under pressure as a crack filler. Finally, the extremely small amount of the acrylamide used does not interfere with the emulsion, so that the emulsion is stable, and the minute amount of acrylamide does not disrupt the final residue film, so that the emulsion film properties approximate the film properties of the original asphaltic component.